1. Field of the Invention
This invention generally relates to an apparatus for monitoring a flame such as a burner flame in a combustion chamber of a boiler, and more particularly, it relates to a monitoring apparatus which is provided with a self-checking function.
2. Description of the Related Art
A monitoring apparatus of this kind is required to be installed in a combustion chamber to give the operator information as to whether the burner is in operation.
U.S. Pat. No. 4,328,488 discloses a flame-monitoring apparatus installed in a combustion chamber. The light emitted from the flame of a particular burner s detected by a light detector in the apparatus, and the status of the burner is monitored.
In order to check if the monitoring apparatus itself is working normally, a periodic shuttering mechanism has been developed. The mechanism shutters periodically the light reaching the detector, and the detected response is compared with the timing or frequency of the shuttering.
This shuttering mechanism may be driven by a reciprocating piston in an air cylinder or by an electromagnetic vibrator. However, the air cylinder requires compressed air and a sophisticated valve mechanism. Furthermore, the reciprocating piston has a problem of friction on its sliding surfaces and the shuttering frequency cannot be sufficiently high. On the other hand, the electromagnetic vibrator requires an electric power source for the electromagnetic coil, and has a problem of heat generation in the coil and of fatigue of the vibrating plate. Therefore, such conventional shuttering mechanisms are not reliable for a sufficiently long time.